Elaine DiMasi

The stomatopod dactyl club: a formidable damage-tolerant biological hammer.

Hammering Home the Lesson Stomatopods are marine crustaceans that use hammerlike claws for defense and to attack their prey. The claws undergo repeated high-velocity and high-force impacts. Weaver et al. (p. 1275; see the Perspective by Tanner) used a variety of techniques to examine the structure, mechanical behavior, and toughening mechanisms of the claw of the Peacock Mantis shrimp. The claws composite structure is optimized for toughness, which helps to prevent the complete failure that might arise from the claws repetitive hammering. The structure of mantis clubs is optimized to prevent complete failure caused by repetitive impacts. Nature has evolved efficient strategies to synthesize complex mineralized structures that exhibit exceptional damage tolerance. One such example is found in the hypermineralized hammer-like dactyl clubs of the stomatopods, a group of highly aggressive marine crustaceans. The dactyl clubs from one species, Odontodactylus scyllarus, exhibit an impressive set of characteristics adapted for surviving high-velocity impacts on the heavily mineralized prey on which they feed. Consisting of a multiphase composite of oriented crystalline hydroxyapatite and amorphous calcium phosphate and carbonate, in conjunction with a highly expanded helicoidal organization of the fibrillar chitinous organic matrix, these structures display several effective lines of defense against catastrophic failure during repetitive high-energy loading events.

Analysis of an ultra hard magnetic biomineral in chiton radular teeth

Recent analyses of the ultrastructural and mechanical properties of mineralized biological materials have demonstrated some common architectural features that can help explain their observed damage tolerance. Nature has accomplished this feat through the precise control of anisotropic crystal nucleation and growth processes in conjunction with nanoscale control over the self-assembly of spatially distinct organic and inorganic phases, resulting in effective inhibition of crack propagation through these materials. One such example is found in the hyper-mineralized and abrasion resistant radular teeth of the chitons, a group of herbivorous marine mollusks who have the surprising capacity to erode away the rocky substrates on which they graze 1-4 . Through the use of modern microscopy and nanomechanical characterization techniques, we describe the architectural and mechanical properties of the radular teeth from Cryptochiton stelleri. Chiton teeth are shown to exhibit the largest hardness and stiffness of any biominerals reported to date, being notably as much as three-fold harder than human enamel and the calcium carbonate-based shells of mollusks. We explain how the unique multi-phasic design of these materials contributes not only to their functionality, but also highlights some interesting design principles that might be applied to the fabrication of synthetic composites.

Surface structure of liquid metals and the effect of capillary waves: X-ray studies on liquid indium

We report x-ray reflectivity ~XR! and small-angle off-specular diffuse-scattering ~DS! measurements from the surface of liquid indium close to its melting point of 156 °C. From the XR measurements we extract the surface structure factor convolved with fluctuations in the height of the liquid surface. We present a model to describe DS that takes into account the surface structure factor, thermally excited capillary waves, and the experimental resolution. The experimentally determined DS follows this model with no adjustable parameters, allowing the surface structure factor to be deconvolved from the thermally excited height fluctuations. The resulting local electron-density profile displays exponentially decaying surface-induced layering similar to that previously reported for Ga and Hg. We compare the details of the local electron-density profiles of liquid In, which is a nearly free-electron metal, and liquid Ga, which is considerably more covalent and shows directional bonding in the melt. The oscillatory density profiles have comparable amplitudes in both metals, but surface layering decays over a length scale of 3.560.6 A for In and 5.5 60.4 A for Ga. Upon controlled exposure to oxygen, no oxide monolayer is formed on the liquid In surface, unlike the passivating film formed on liquid gallium. @S0163-1829~99!11701-6#

Molecular Crowding of Collagen: A Pathway to Produce Highly-Organized Collagenous Structures

Collagen in vertebrate animals is often arranged in alternating lamellae or in bundles of aligned fibrils which are designed to withstand in vivo mechanical loads. The formation of these organized structures is thought to result from a complex, large-area integration of individual cell motion and locally-controlled synthesis of fibrillar arrays via cell-surface fibripositors (direct matrix printing). The difficulty of reproducing such a process in vitro has prevented tissue engineers from constructing clinically useful load-bearing connective tissue directly from collagen. However, we and others have taken the view that long-range organizational information is potentially encoded into the structure of the collagen molecule itself, allowing the control of fibril organization to extend far from cell (or bounding) surfaces. We here demonstrate a simple, fast, cell-free method capable of producing highly-organized, anistropic collagen fibrillar lamellae de novo which persist over relatively long-distances (tens to hundreds of microns). Our approach to nanoscale organizational control takes advantage of the intrinsic physiochemical properties of collagen molecules by inducing collagen association through molecular crowding and geometric confinement. To mimic biological tissues which comprise planar, aligned collagen lamellae (e.g. cornea, lamellar bone or annulus fibrosus), type I collagen was confined to a thin, planar geometry, concentrated through molecular crowding and polymerized. The resulting fibrillar lamellae show a striking resemblance to native load-bearing lamellae in that the fibrils are small, generally aligned in the plane of the confining space and change direction en masse throughout the thickness of the construct. The process of organizational control is consistent with embryonic development where the bounded planar cell sheets produced by fibroblasts suggest a similar confinement/concentration strategy. Such a simple approach to nanoscale organizational control of structure not only makes de novo tissue engineering a possibility, but also suggests a clearer pathway to organization for fibroblasts than direct matrix printing.

Phase identification of self-forming Cu–Mn based diffusion barriers on p-SiOC:H and SiO2 dielectrics using x-ray absorption fine structure

X-ray absorption fine structure spectroscopy has been used to study the chemical and structural properties of self-forming diffusion barrier layers from Cu-8 at. % Mn alloy films on porous low-k and thermally grown SiO2 dielectrics. For the porous low-k/Cu(Mn) system, we provide evidence that the interface is composed of MnSiO3 and MnO with near complete Mn segregation from the alloy film; however, we find that the self-forming process does not go to full completion on thermally grown SiO2 substrates.

Short-Range Smectic Order in Bent-Core Nematic Liquid Crystals

Small angle X-ray diffraction from the uniaxial nematic phase of certain bent-core liquid crystals is shown to be consistent with the presence of molecular clusters possessing short-range tilted smectic (smectic-C) order. Persistence of these clusters throughout the nematic phase, and even into the isotropic state, likely accounts for the unusual macroscopic behavior previously reported in bent-core nematics, including an anomalously large flexoelectric effect (∼ 1000 times that of conventional calamitic nematics), very large orientational and flow viscosities (∼ 10–100 and ∼ 100–1000 times, respectively, typical values for calamitics), and an extraordinary flow birefringence observed in the isotropic state.

Morphology of air nanobubbles trapped at hydrophobic nanopatterned surfaces.

The details of air nanobubble trapping at the interface between water and a nanostructured hydrophobic silicon surface are investigated using X-ray scattering and contact angle measurements. Large-area silicon surfaces containing hexagonally packed, 20 nm wide hydrophobic cavities provide ideal model surfaces for studying the morphology of air nanobubbles trapped inside cavities and its dependence on the cavity depth. Transmission small-angle X-ray scattering measurements show stable trapping of air inside the cavities with a partial water penetration of 5-10 nm into the pores, independent of their large depth variation. This behavior is explained by consideration of capillary effects and the cavity geometry. For parabolic cavities, the liquid can reach a thermodynamically stable configuration-a nearly planar nanobubble meniscus-by partially penetrating into the pores. This microscopic information correlates very well with the macroscopic surface wetting behavior.

Hybrid Langmuir–Blodgett monolayers containing clay minerals: effect of clay concentration and surface charge density on the film formation

To control the properties of hybrid organo-clay films prepared by the Langmuir–Blodgett (LB) method, the film formation mechanism should be understood. This work aimed to understand what occurs at the air–water interface after the spreading of cationic surfactants (octadecyl rhodamine B, 3,3′-dioctadecyl oxacarbocyanine) on aqueous dispersions of smectite clay minerals (saponite, montmorillonite, hectorite, laponite), resulting in hybrid organo-clay films. Information on the amount of surfactant molecules and clay particles in the hybrid films was obtained with surface pressure versus molecular area isotherms, attenuated total reflection infrared spectroscopy, ultraviolet-visible spectroscopy and atomic force microscopy. X-ray reflectivity measurements indicated that the surfactant molecules had adsorbed on only one side of the clay mineral lamella. With increasing clay concentration of the dispersion, the isotherms shifted to a lower lift-off area, a minimum lift-off area (MLA) was reached and then the lift-off area increased again. Films made at lower than the MLA clay concentration consisted of two phases: an organic phase and a hybrid organo-clay phase. Films made at the MLA clay concentration consisted of dense monolayers of the surfactant molecules and single clay mineral lamellae. The density of the surfactant molecules was highly correlated with the surface charge density of the clay minerals. These films had low water content. Films made at higher than the MLA clay concentration contained less surfactant, aggregates of the clay mineral particles, residual Na+ ions and water. With the clay concentration of the dispersion and the surface charge density of the clay mineral, the properties of hybrid organo-clay LB films can be adjusted.

Microscopic structure of the wetting film at the surface of liquid Ga-Bi alloys

X-ray reflectivity measurements of the binary liquid Ga-Bi alloy reveal a dramatically different surface structure above and below the monotectic temperature

Oriented hydroxyapatite in turkey tendon mineralized via the polymer-induced liquid-precursor (PILP) process

T_{mono}=222^{\circ}